Stent

ABSTRACT

Expandable stent for insertion into a body passage having a mesh structure of interconnecting portions ( 6 ) joined together by joining portions ( 5 ). The stent, when inserted into said body passage, is adapted to dissolve into smaller parts, wherein the joining portions dissolve faster than the interconnecting portions. In a preferred embodiment the joining portions are made from a first material and the interconnecting portions are made from a second material different from said first material, wherein the first material dissolves faster than said second material.

FIELD OF THE INVENTION

The present invention relates to an expandable stent according to thepreamble of the independent claim.

The present invention relates generally to the field of expandablestents for insertion into vessels in the body, and particularly todissolvable stents being made of a metal that dissolves by corrosioninside the body vessel and to disintegrating stents being made of twometals with different electrochemical potentials, thereby forming agalvanic element in which an electrochemical reaction occurs thatconsumes the metal having the lower electrochemical potential.

BACKGROUND OF THE INVENTION

A number of different stents have been proposed for the stenting of ablood vessel that has been occluded. A widely used type of stentconsists of an expandable metal mesh. This type of stent may be furtherdivided into self-expanding stents and non-self-expanding stents.

The self-expanding stents can be made of a mesh material that changes toa larger-size configuration upon heating to body temperature. Examplesof stents of this type may be found in U.S. Pat. No. 6,071,308. Otherself-expanding mesh stents are made of a resilient material, which canbe flexed down into a small diameter tube and held in place in thisconfiguration until it is released, at which time the mesh expands tothe larger configuration.

The non-self-expanding stents are often expanded by use of an inflatableballoon, which is placed inside the mesh being in the small diameterconfiguration and which is then inflated, thereby expanding the mesh tothe large diameter configuration. The balloon itself is then deflatedfor removal, while the metal mesh is left in the expanded configuration.For examples of non-self-expanding stents, see U.S. Pat. No. 5,799,384and the international application WO-0189417.

Some of these expandable metal mesh stents are combined with anexpandable polymer layer, which may be positioned on the inside of theexpandable mesh, on the outside of the expandable mesh, within theinterstices of the expandable mesh, or any combination of inside,outside and within the interstices of the expandable mesh stent. A stentof this type is, for example, shown in U.S. Pat. No. 5,968,070, whereinthe polymer layer may consist of expanded polytetrafluoroethylene(PTFE). As disclosed in, for example, U.S. Pat. No. 5,160,341, it isalso possible to use a polymer layer made of a resorbable polymer, suchas polylactic acid homopolymers, polyglycolic acid homopolymers, orcopolymers of polylactic acid and polyglycolic acid.

One advantage with expandable metal mesh stents is that their smalldiameter in the pre-expanded state allows easy insertion into narrowvessels. However, after the expansion, the metal mesh stents aredifficult to remove since tissue in-growth occurs over time, and, inpractise, the stents are normally left inside the blood vessel. The maincomplication associated with the stenting of a stenosis in a bloodvessel is the risk of having a restenosis, in which case a new stenosisdevelops at the same position as the first one, i.e. a new stenosis isgrowing inside the inserted stent. Several types of stents have beensuggested to handle this severe problem, including drug-deliveringstents and radioactive stents. Examples of drug-delivering stents may befound in U.S. Pat. No. 6,206,195, while examples of stents forradiotherapy may be found in U.S. Pat. No. 6,192,271.

Nevertheless, there is still a substantial risk of having a restenosisfollowing the stenting of a coronary artery. In this case, a secondstent is normally inserted and expanded inside the first one, whichobviously reduces the diameter of the second stent in its expandedconfiguration as well as the inner diameter of the re-stented bloodvessel.

Further, when a stent is placed permanently inside a coronary artery,the continuous stress from the beating of the heart may cause the walland edges of the stent to damage the vessel wall. This damage can leadto arterial rupture or aneurysm formation. Also, a stent adapted to bepermanently implanted within a blood vessel is continuously exposed tothe flow of blood inside the vessel, which may lead to thrombusformation within the blood vessel. Stents made of absorbable materials(see e.g. U.S. Pat. No. 5,306,286) have been proposed in order toovercome these problems. A disadvantage with such stents is that theyare difficult to expand, i.e. they are of the self-expandable type. Theyhave also a limited capability to withstand the compressive pressureexerted by the blood vessel in their expanded configuration.

A biodegradable polymeric stent having a programmed pattern of in vivodegradation is disclosed in U.S. Pat. No. 5,957,975. The stent comprisesa substantially cylindrical element having two open ends and a pluralityof different regions where each region has a desired in vivo lifetime.

And finally, U.S. Pat. No. 6,287,332 discloses an implantable,bioresorbable vessel wall support, in particular a coronary stent, thatcomprises a combination of metal materials which dissolves in the humanbody without any harmful effects on the person that wears the implant.The combination of metal materials can be an alloy or a local galvanicelement. No specific structure of the stent is disclosed in U.S. Pat.No. 6,287,332.

It would therefore be desirable to provide a stent that combines theexpandability and structural integrity of the metal mesh stents with theadvantages of the absorbable stents. Such a stent would allow easyinsertion into the blood vessel and yet being expandable enough toexpand the blood vessel to the desired volume. The stent should alsoavoid the complications associated with permanently implanted stents bybecoming dissolved or disintegrated. A stent having thesecharacteristics would allow stenting of a restenosis, with the finalinner diameter of the re-stented blood vessel being the same as afterthe first stenting operation.

Furthermore, a blood vessel provided with a stent in the longer termwill loose some of its elasticity. An absorbable stent that relativelyquickly and in a controllable manner looses its mechanical strengthwould enable, in an advantageous way, the blood vessel to rapidly regainits elasticity.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an expandable stent,which dissolves or disintegrates inside a blood vessel after apredefined time.

In a first embodiment, the stent comprises a mesh made of a metal thatdissolves by corrosion in the environment prevailing within the bloodvessel.

In a second embodiment, the metal mesh is made of at least two metalshaving different electrochemical potentials, thereby forming an activegalvanic element. In the galvanic element, an electrochemical reactionoccurs, which consumes the metal having the lower electrochemicalpotential. If the joints of the metal mesh are made of the metal havingthe lower electrochemical potential, these joints will dissolve, whichleaves the rest of the mesh in a disintegrated configuration.

In a third embodiment, a more generalised stent, is disclosed, having amesh structure of interconnecting parts joined together by joining partsthat, when inserted into said body passage, is adapted to dissolve intosmaller parts, wherein the joining parts dissolves faster than theinterconnecting parts. Preferably, the joining parts and theinterconnecting parts are made from a first and a second material,respectively, wherein the first material dissolves faster than thesecond material.

The above-mentioned object is achieved by an expandable stent and by amethod according to the independent claims.

Preferred embodiments are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of an expandable metal mesh stentaccording to the invention.

FIG. 2 shows a second embodiment of an expandable metal mesh stentaccording to the invention.

FIG. 3 shows the stent of FIG. 2 in a disintegrated state.

FIG. 4 shows a fourth embodiment of an expandable metal mesh stentaccording to the invention.

FIG. 5 shows a cross-section of the stent of FIG. 4.

FIG. 6 a illustrates a modification of a third embodiment of the presentinvention.

FIGS. 6 b and 6 c illustrate the third embodiment of the presentinvention.

FIGS. 7 a-7 c illustrate a variant of the third embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a first embodiment of an inventive stent. In FIG. 1,a stent 1 comprises a mesh 2 made of metal that corrodes in theenvironment prevailing inside a vessel. By choosing a suitable metal, itis possible to control the time elapsed until the stent is dissolved bycorrosion inside the vessel. Obviously, this time depends on thephysiological and chemical characteristics of both the vessel itself andthe fluid flowing inside the vessel as well as for how long time it isnecessary to support the stented vessel. A perhaps natural choice ofmetal would in this case be iron, or possibly an alloy of iron and asmall amount of chromium or nickel in order to make the stent moreresistant to corrosion, i.e. prolong the time before the stent isdissolved inside the vessel. In practise, the choice of metal or alloymay be tailored to the actual application.

A second embodiment of an inventive stent is illustrated in FIG. 2.Here, a stent 3 comprises a metal mesh 4, which comprises two componentparts, joining 5 and interconnecting portions 6. If the joints 5 aremade of metal having a lower electrochemical potential than the metal ofthe interconnecting portions 6, an active galvanic element is created,with the fluid inside the vessel acting as an electrolyte. This galvanicelement drives an electrochemical process, in which the metal having thelower electrochemical potential is consumed, which, in this case, meansthat the joining portions 5 of the mesh 4 are dissolved, thereby leavingthe mesh 4 in a disintegrated configuration. This disintegratedconfiguration is shown in FIG. 3. As is well known, the kinetics ofcorrosion reactions may in actual practise differ from that predicted byelectrochemical potentials in standard electrochemical series. Whendeciding metal combinations, one must therefore also take into accountthe characteristics of the vessel in question.

In the second embodiment described above, the joining portions 5 could,for example, consist of zinc while the interconnecting portions 6consist of iron. With this material combination, the whole stent 3 wouldeventually be dissolved since the interconnecting portions 6 woulddissolve by corrosion when the joining portions 5 have been consumed inthe electrochemical process of the galvanic element. Another possibilityis to make the mesh 4 of a first metal, such as iron, and then provide alayer of a second metal, such as gold, having a higher electrochemicalpotential at the joining portions 5. This configuration would create anelectrochemical process in which the first metal (e.g. iron) is consumedbeneath the layer of the second metal (e.g. gold). This combinationwould yield the same disintegrated configuration as shown in FIG. 3, theonly difference being the small remainders of the second metal at thejoining portions 5. In practise, the remaining amounts of the secondmetal can be made negligible small. As before, the specific materialsand material combinations can be tailored to the desired time beforedisintegration of the stent. It is, of course, also possible to providethe two metals at other positions than the joints and straight portionsof the mesh, which would leave the disintegrated stent in some otherconfiguration than the one shown in FIG. 3. Further, the metal meshcould be made of more than two metals with different electrochemicalpotentials. If, for example three metals were used, two differentgalvanic elements would be created, which provides additionalpossibilities to adapt the disintegrations rates of the metal meshes aswell as the disintegrated configurations to the specific applicationconditions.

A more general third embodiment of the present invention is illustratedby FIGS. 2 and 3, and by FIGS. 6 a-6 c and 7 a-7 c.

By using the same reference signs, in FIGS. 6 a-6 c and 7 a-7 c, as inFIG. 2 the generalised expandable stent 3 comprises a mesh structure 4of interconnecting portions 6 joined together by joining portions 5.

According to this third embodiment the joining portions are made from afirst material and the interconnecting portions are made from a secondmaterial different from said first material, wherein the first materialdissolves faster than said second material.

By using this mesh structure the stent dissolves in such a way that thelongitudinal structural integrity initially is decreased and that thelongitudinal structural integrity decreases faster than the radialstructural integrity decreases. The radial structural integrity isrelated to the forces exerted by the stent towards the body passagewall. Thereby the flexibility of the stent in the longitudinal directionof the body passage gradually increases but the support of the bodypassage wall remains more or less unchanged for a longer time.

FIGS. 6 a-6 c and 7 a-7 c illustrate the degrading procedure.

In FIGS. 6 a-6 c a stent having a mesh structure comprising hexagonalcells is illustrated.

In a modified third embodiment illustrated in FIG. 6 a some of theinterconnecting portions 6′ are made from the same material as thejoining portions. These modified interconnecting portions 6′ are locatedin a plane perpendicular to the main direction of the body passage andby arranging a number of these ring-shaped structures along the stent,the flexibility of the stent in the longitudinal direction is therebyincreased. The support of the body passage wall remains more or lessunchanged for a longer time due to the remaining smaller parts. Thedistances between the ring-shaped structures of joining portionsinfluence the sizes of the smaller parts into which the stent initiallydegrades. These remaining smaller parts may then have an essentiallycylindrical shape, but they may also be more or less ring-shaped.

FIGS. 6 b and 6 c illustrate the third embodiment with no modifiedinterconnecting portions where the joining portions just have beendissolved (FIG. 6 b) and where the structure of the interconnectingportions 6 of the stent is more or less broken (FIG. 6 c).

According to a preferred alternative of this third embodiment thejoining portions are made from a said first material that is aresorbable polymer.

Although illustrated as essentially being straight the interconnectingportions may also have other shapes, e.g. being curve-shaped.

In this third embodiment the joining portions and interconnectingportions may also be made of different metals, a first metal and asecond metal, respectively. These metals have different electrochemicalpotentials, thereby forming a galvanic element that drives anelectrochemical process in which the first metal is consumed inside saidbody passage leaving the stent in smaller parts, which have cylindricalshapes or a ring shapes.

In FIGS. 7 a-7 c a stent having a mesh structure comprising quadratic orrectangular cells is illustrated. In FIG. 7 a the stent, comprisinginterconnecting portions 6 and joining portions 5, is shown priorimplantation, and in FIG. 7 b the degrading procedure has come to thepoint where the joining portions have degraded but the interconnectingportions are left more or less unchanged. In the phase of the degradingprocedure illustrated in FIG. 7 c the remaining structure of theinterconnecting portions of the mesh structure are beginning to bedissolved.

The invention also relates to a method of manufacturing stents by directlaser cutting from a single metal tube. For the inventive purposes, thismethod could be applied on a tube made of two metals. FIG. 4 illustratesa stent 7, which has been laser-cut to a desired mesh structure 8. As isshown in cross-section in FIG. 5, the stent 7 is made from a metal tubecomprising a first layer 9 of first metal, such as stainless steel, anda second layer 10 of a second metal, such as platinum, the second metalhaving a higher electrochemical potential than the first metal. Forclarity of illustration, the two layers have been enlarged in FIG. 5. Inpractise, the second metal would have been applied as a very thin layer10 on the outside of the tube. As an alternative, the second metal couldbe applied on the inside of the tube. With this configuration, lasercutting or other conventional manufacturing methods, such as etching,can be applied as for a stent made from a single metal tube.Furthermore, such a stent would exhibit essentially the same mechanicalproperties, as a stent made of the first metal only. The latter is, ofcourse, only valid before and immediately after implantation in avessel, i.e. before the start of any electrochemical process.

In this context, it should be noted that the normal corrosion processalso is an electrochemical process, and if two or more metals are usedin a stent, one (or all) of the metals will corrode and dissolve due tothe normal corrosion mechanism, in addition to the corrosion driven bythe galvanic element as described above. It should also be noted that itis possible to obtain “internal” galvanic elements if granules or smallcells of a second metal are present in a first metal. The second metalmay be present naturally in the first metal or may be implanted into thefirst metal by means of some suitable technique such as sintering.Obviously, the same effect would arise if the metal of which the stentis made comprises more than two metals with different electrochemicalpotentials. Such internal galvanic elements would accelerate the normalcorrosion process and would also provide a further possibility tocontrol the disintegration of the stent. With appropriate choice ofmetals, the same effect may also be utilized if an alloy or a compoundof two or more metals is used for the manufacturing of the stents.

Finally, it should be noted that herein the term “expandable”encompasses both self-expanding and non-self-expanding mesh stents.

Although the present invention has been described with reference tospecific embodiments, also shown in the appended drawings, it will beapparent for those skilled in the art that many variations andmodifications can be done within the scope of the invention as describedin the specification and defined in the following claims.

1. Expandable stent for insertion into a body passage having a meshstructure of interconnecting portions (6) joined together by joiningportions (5), characterized in that said stent, when inserted into saidbody passage, is adapted to dissolve into smaller parts, wherein thejoining portions dissolves faster than the interconnecting portions. 2.Expandable stent according to claim 1, characterized in that the andthat the joining portions are made from a first material and theinterconnecting portions are made from a second material different fromsaid first material, wherein the first material dissolves faster thansaid second material.
 3. Expandable stent according to claim 1,characterized in that the mesh structure makes the stent to dissolve insuch a way that the longitudinal structural integrity initially isdecreased.
 4. Expandable stent according to claim 3, characterized inthat the longitudinal structural integrity decreases faster than theradial structural integrity decreases.
 5. Expandable stent according toclaim 4, characterized in that the radial structural integrity isrelated to the forces exerted by the stent towards the body passagewall.
 6. Expandable stent according to claim 1, characterized in thatsaid smaller parts have an essentially cylindrical shape.
 7. Expandablestent according to claim 1, characterized in that said smaller parts areessentially ring-shaped.
 8. Expandable stent according to claim 1,characterized in that said first material is a resorbable polymer. 9.Expandable stent according to claim 1, characterized in that saidinterconnecting portions being straight.
 10. Expandable stent accordingto claim 1, characterized in that said interconnecting portions beingcurve-shaped.
 11. Expandable stent according to claim 1, characterizedin that said joining portions are made of metal.
 12. Expandable stentaccording to claim 1, characterized in that said interconnectingportions are made of metal.
 13. Expandable stent according to claim 1,characterized in that said joining portions and interconnecting portionsare made of different metals, a first metal and a second metal,respectively.
 14. Expandable stent (3; 7) according to claim 13,characterized in that said first and second metals have differentelectrochemical potentials, thereby forming a galvanic element thatdrives an electrochemical process in which the first metal is consumedinside said body passage.
 15. Expandable stent (3; 7) according to claim14, characterized in that the first metal is consumed in saidelectrochemical process after a pre-defined time inside said bodypassage.
 16. Expandable stent (3; 7) according to claim 14,characterized in that the second metal dissolves by corrosion insidesaid body passage.
 17. Expandable stent (3; 7) according to claim 14,characterized in that the second metal dissolves by corrosion after apre-defined time inside said body passage.
 18. Expandable stent (7)according to claim 13, characterized in that the second metal isprovided as a thin layer on the first metal.
 19. Expandable stent (7)according to claim 13, characterized in that the second metal isprovided as a thin layer on selected parts of the first metal. 20.Expandable stent according to claim 13, characterized in that the secondmetal is provided as granules or cells within the first metal. 21.Expandable stent according to claim 13, characterized in that the firstmetal and the second metal are in the form of an alloy or a compound.22. Expandable stent according to claim 1, characterized in that thestent comprises more than two metals, all of which have differentelectrochemical potentials, thereby forming galvanic elements that eachdrives a respective electrochemical process in which the metal havingthe lower electrochemical potential is consumed.
 23. Expandable stentaccording to claim 1, characterized in that the joining portions and theinterconnecting portions are made from the same material.
 24. Expandablestent according to claim 23, characterized in that said material is ametal.
 25. Expandable stent according to claim 24, characterized in thatsaid metal dissolves by corrosion inside said body passage. 26.Expandable stent according to claim 23, characterized in that thejoining portions dissolve faster than the interconnecting portions. 27.Expandable stent according to claim 26, characterized in that thejoining portions have a higher porosity compared to the interconnectingportions.
 28. Expandable stent according to claim 26, characterized inthat joining portions have a smaller radial thickness as compared to theradial thickness of the interconnecting portions.
 29. Expandable stent(1; 3; 7) according to claim 24, characterized in that said metaldissolves by corrosion after a pre-defined time inside said bodypassage.
 30. Method for the manufacturing of an expandable metal stentfor insertion into a body passage having a mesh structure ofinterconnecting portions (6) joined together by joining portions (5),said stent, when inserted into said body passage, is adapted to dissolveinto smaller parts, wherein the joining portions dissolves faster thanthe interconnecting portions, the stent comprises a first metal and asecond metal, the second metal having an electrochemical potential thatdiffers from the electrochemical potential of the first metal, andwherein said method includes that the metal stent (7) is made from atube of the first metal, the outer surface and/or the inner surface ofthe tube being coated with a layer of the second metal.
 31. Methodaccording to claim 30, further characterized in that the tube, which ismade of the first metal, is coated with layers of several metals, all ofwhich have different electrochemical potentials.
 32. Method according toclaim 30, characterized in that said manufacturing involves lasercutting or etching.